The idea behind the load-linked/store-exclusive paradigm is that if if the store follows very soon after the load, with no intervening memory operations, and if nothing else has touched the location, the store is likely to succeed, but if something else has touched the location the store is certain to fail. There is no guarantee that stores will not sometimes fail for no apparent reason; if the time between load and store is kept to a minimum, however, and there are no memory accesses between them, a loop like:
do
{
new_value = __LDREXW(dest) + 1;
} while (__STREXW(new_value, dest));
can generally be relied upon to succeed within a few attempts. If computing the new value based on the old value required some significant computation, one should rewrite the loop as:
do
{
old_value = *dest;
new_value = complicated_function(old_value);
} while (CompareAndStore(dest, new_value, old_value) != 0);
... Assuming CompareAndStore is something like:
uint32_t CompareAndStore(uint32_t *dest, uint32_t new_value, uint_32 old_value)
{
do
{
if (__LDREXW(dest) != old_value) return 1; // Failure
} while(__STREXW(new_value, dest);
return 0;
}
This code will have to rerun its main loop if something changes *dest while the new value is being computed, but only the small loop will need to be rerun if the __STREXW fails for some other reason [which is hopefully not too likely, given that there will only be about two instructions between the __LDREXW and __STREXW]
Addendum
An example of a situation where "compute new value based on old" could be complicated would be one where the "values" are effectively a references to a complex data structure. Code may fetch the old reference, derive a new data structure from the old, and then update the reference. This pattern comes up much more often in garbage-collected frameworks than in "bare metal" programming, but there are a variety of ways it can come up even when programming bare metal. Normal malloc/calloc allocators are not generally thread-safe/interrupt-safe, but allocators for fixed-size structures often are. If one has a "pool" of some power-of-two number of data structures (say 255), one could use something like:
#define FOO_POOL_SIZE_SHIFT 8
#define FOO_POOL_SIZE (1 << FOO_POOL_SIZE_SHIFT)
#define FOO_POOL_SIZE_MASK (FOO_POOL_SIZE-1)
void do_update(void)
{
// The foo_pool_alloc() method should return a slot number in the lower bits and
// some sort of counter value in the upper bits so that once some particular
// uint32_t value is returned, that same value will not be returned again unless
// there are at least (UINT_MAX)/(FOO_POOL_SIZE) intervening allocations (to avoid
// the possibility that while one task is performing its update, a second task
// changes the thing to a new one and releases the old one, and a third task gets
// given the newly-freed item and changes the thing to that, such that from the
// point of view of the first task, the thing never changed.)
uint32_t new_thing = foo_pool_alloc();
uint32_t old_thing;
do
{
// Capture old reference
old_thing = foo_current_thing;
// Compute new thing based on old one
update_thing(&foo_pool[new_thing & FOO_POOL_SIZE_MASK],
&foo_pool[old_thing & FOO_POOL_SIZE_MASK);
} while(CompareAndSwap(&foo_current_thing, new_thing, old_thing) != 0);
foo_pool_free(old_thing);
}
If there will not often be multiple threads/interrupts/whatever trying to update the same thing at the same time, this approach should allow updates to be performed safely. If a priority relationship will exist among the things that may try to update the same item, the highest-priority one is guaranteed to succeed on its first attempt, the next-highest-priority one will succeed on any attempt that isn't preempted by the highest-priority one, etc. If one was using locking, the highest-priority task that wanted to perform the update would have to wait for the lower-priority update to finish; using the CompareAndSwap paradigm, the highest-priority task will be unaffected by the lower one (but will cause the lower one to have to do wasted work).
